Lecture 6

Question 1

4 types of defects, what do defects influence ?

Answer 1

point defects
line defects
2D defects
3D defects (bulk).

Defects have a big impact on the properties of the material

Question 2

4 types of 0D defects, what do they lead to ? consequence ?

Answer 2

1) vacancy : missing atom
2) interstitial : not on lattice point
3) large substitution : foreign material
4) small substitution : foreign material

Lattice planes are deformed -> stiffness of material increases

Question 3

vacancies : what kind of defect, formula at thermal equilibrium, entropy ?

Answer 3

thermal defects : number depends on T (higher T = more vacancies).
c_v = exp(-deltaU / R*T).

more vacancies = higher entropy

Question 4

what does the concentration of vacancies influence ?

Answer 4

• Thermally activated processes such as diffusion
• segregation
• plastic deformation

Question 5

0D defects in ionic crystals : stoichiometric lattices and non-stoichiometric lattices

Answer 5

1) stoichiometric (all of same concentration)
- schottky : vacancies on lattice sites
- anti-schottky : vacancies on interstitials
- frenkel : metal vacancies and interstitials
- anti-frenkel : non-metal vacancies and interstitals
- anti-structural disorder : metal on non-metal sires and non-metal on metal sites

2) non-stoichiometric : electrons are redistributed resulting in conductivity -> changes diffusion, deformation, optical properties, …

Question 6

dislocations : what can generate them, two types, what characterizes them ?

Answer 6

• crystal formation, stress / T gradients, stresses at higher dimensional effects
• edge and screw
• burgers vector b : magnitude and direction of lattice distorsion resulting from a dislocation

Question 7

edge dislocation : what is it, burgers vector

Answer 7

Additional half lattice plane.
Vector is perpendicular to dislocation line.

Question 8

screw disloc : vector ?

Answer 8

vector is parallel to dislocation line (step height)

Question 9

vector when combined disloc ?

Answer 9

the vector is neither parallel nor perpendicular to disloc line -> bent

Question 10

how can a disloc move ? in what direction ? exception ?

Answer 10

Under application of force / stress. Gliding only happens in planes, determined by direction of burgers vector.

Only exception is screw disloc : can change the glide plane (cross-gliding)

Question 11

climbing of dislocations

Answer 11

Dislocations move by absorbing or emitting vacancies. When a dislocation absorbs vacancies, it moves upwards, away from the obstacle, and then re-emits vacancies to continue its motion.

strongly dependent on T

Question 12

dislocation jogs

Answer 12

happens when two dislocations interact
(jogs of two screw dislocs are an edge disloc -> hindrance of future movement)

Question 13

stress and strain in dislocation networks, how can disloc vanish ?

Answer 13

Compressive and tensile stresses -> defect clustering.
Edge disloc can vanish if they are opposite type

Question 14

energy to build a screw disloc ? why do they not exist in thermodynamic equilibrium ?

Answer 14

E = Gb^2/4pi * ln(r1/r0)

G = shear modulus (big value)
r0 = radius of disloc nucleus (3 A)
r1 = half mean distance of disloc in solid state (1 micron)

They don’t exist because they are non energetically favorable (in equilibrium, the crystal wants to be at minimal energy state)

Question 15

burgers vectors in simple lattices

Answer 15

fcc : b = a/2<110>
bcc: b = a/2<111> or a<100>
hcp: b = a<11-20> or b = c<0001>

Question 16

2D lattice defects : what, how can they be formed

Answer 16

Stacking faults : created during crystal formation or formed out of dislocations

Question 17

what is a partial disloc ?

Answer 17

A partial dislocation has a Burgers vector that is shorter than a unit lattice translation vector for the crystal structure.

Question 18

3D defects, angle ?

Answer 18

Grain boundaries with slightly misoriented grains (mosaic).

alpha = a/d when a = interatomic distance, d = distance between edge dislocs

Question 19

order of increased defect densities

Answer 19

single crystal < polycrystalline < nanocrystalline < amorphous